restored run-time thread lib detection

sql/stacktrace.c:
  removed code duplication
sql/stacktrace.h:
  removed code duplication

include/my_pthread.h

@@ -701,6 +701,15 @@ extern uint my_thread_end_wait_time;
   Keep track of shutdown,signal, and main threads so that my_end() will not
   report errors with them
 */
+
+/* Which kind of thread library is in use */
+
+#define THD_LIB_OTHER 1
+#define THD_LIB_NPTL  2
+#define THD_LIB_LT    4
+
+extern uint thd_lib_detected;
+
 	/* statistics_xxx functions are for not essential statistic */
 
 #ifndef thread_safe_increment